Heavy duty electrical power motors and generators used to drive machinery as primary or back-up electrical power source, and particularly synchronous electrical motors and generators are subject to sudden variations in the magnetic flux between the rotor and stator windings everytime there is a phase shift between the rotor armature flux and the stator armature flux. Such a phase shift occurs upon sudden increase or decrease in the mechanical load or in the power fed to a synchronous motor, and sudden increase or decrease in the electrical load or the torque applied to a synchronous generator.
The sudden phase shift can produce substantial voltage transients that cannot be tolerated by a critical load of a generator, or that can be fed back on the line power to a motor.
Sensitive loads such as data processing equipment and medical instruments must nowadays be protected by heavy, cumbersome and costly filtering and regulating devices. Utilities must protect their installations against power surges fed back by a motor suddenly turned into a generator by use of circuit breakers and other safety devices which increase the risk of blackouts.
The instant invention results from an attempt to limit these undesirable spurious lines transients by attacking the problem at its source, i.e. inside the synchronous power machinery.
Uninterruptible power supply systems are provided for switching from a normal line supply of electric power to a standby or backup power supply in the event of interruption, failure or inadequacy of the normal power supply. For true uninterruptible power, the switching must be virtually instantaneous, and for cyclic power, without any change in phase, frequency or voltage.
Uninterruptible power supplies may require a transitional supply of power, such as a flywheel, to carry the load until the standby source of power can be brought up to speed and switched on-line. The flywheel may have to be combined with fast acting power loss sensors and solid state switches in order to keep flywheel weight and size reasonable. An example of a fast acting switch is shown in U.S. Pat. No. 4,827,152.
Many of the current uninterruptible power supplies may accomplish the switching quickly, but not without a significant flywheel weight. Depending upon rotating mass requirements, the flywheel may be a separate structure or incorporated in the construction of the motor, shaft, or rotor. For critical loads where even a small change in frequency can cause problems, flywheel size requirement may become unpractical. The flywheel large spinning mass also creates problems in startup and performance when acceleration or deceleration of the spinning mass is required.
Even fairly massive flywheels and fast acting switches may have to be supplemented by spinning stand-by, dual drives, and other devices. An example of additional devices required is shown in U.S. Pat. No. 3,458,710.
Holding cyclic electric power frequency and phase within tolerances for these additional devices for critical loads has also been a problem. Frequency changers (e.g., coupled rotating units at different phases at any one rotary position) and other Complex devices have been used to control critical frequency and phase outputs from rotating electric machines for critical applications.
There is a need for a simpler and more effective power backup system which can instantaneously detect any failure of the line power and synchronously switch to a stand-by power supply without resorting to massive kinetic energy storage device or complex phase and frequency maintenance circuitry.